1. Field of the Invention
The present invention relates to an illumination source for passive millimeter-wave imaging.
2. Description of the Art
Many available imagers for security applications can produce images which enable an operator to readily detect concealed weapons such as guns and knives, which are hidden for example within a person's clothing or baggage. Clothing and baggage materials are virtually transparent to at least some of these known imagers and this may be advantageous when metallic objects are solely of interest, since they will not be obscured by non-metallic material.
This does mean, however, that such imagers are not capable of providing recognisable images of passive non-metallic objects such as plastics, ceramics and explosives, which nowadays are often of more interest.
Millimeter-wave imaging addresses this problem. In outdoor passive millimeter-wave imaging high contrast is provided in the generated image by cold sky illumination. Depending on the geometry of the viewed scene, materials such as metals can reflect this illumination towards the imager, appearing cold and exhibiting a high contrast against the generally warm background. In addition, however, it is possible to use millimeter-wave imaging for detecting passive non-metallic objects, and this technique also allows the remote and covert scanning of suspects.
Passive millimeter-wave imaging can be accomplished indoors but the lack of sky illumination means that the main source of contrast is now the actual temperature difference between objects. This contrast will be of the order of 10K, which is an order of magnitude less than what can be expected in outdoor imagery.
Another alternative is to use an artificial source of millimeter-wave radiation, to illuminate the area being imaged in order to improve the contrast in the generated image. The relatively long wavelength of millimeter-wave radiation means that many reflections from visibly rough or dull surfaces are specular in nature in the millimeter-wave portion of the electromagnetic spectrum, i.e. many visibly rough or dull surfaces behave similarly to a mirror to millimeter-wave radiation. This effect is noticeable when a person being imaged is illuminated from a small source. In this situation the person does not appear uniformly warm in the generated image, but instead warm glints appear on the body of the person where specular reflection of the source from the body is incident on the aperture of the imager. This effect makes the generated image difficult to interpret in real time, particularly in real time.
This problem can be overcome by locating large areas of radar absorbent material, for example over the walls, ceiling and floor of an indoor area in which passive millimeter-wave imaging is to be done and heating or cooling the radar absorbent material to a temperature that is different from the ambient temperature of the objects in the indoor area that are being imaged. Alternatively, large area portable panels of heated or cooled radar absorbent material can be set up in the indoor area in which imaging is to take place. This approach requires a lot of energy to heat or cool the large area of radar absorbent material and does not lend itself to portability.
It would therefore be desirable to provide a non-directional illumination source of millimeter-wave radiation, where the radiation also has a low degree of coherence, or no coherence, and which has relatively low power consumption. It would also be desirable to provide such an illumination source in the form of a uniformly radiating surface. Ideally the radiating surface would appear to have the same brightness at all angles of observation, so approximating a black body radiator.